For the manufacture of such compositions as the binder use is predominantly made of water-soluble resol phenol-formaldehyde resins or mixtures thereof with phenolalcohols, as well as carbamide resins. As the filler use is made mainly of crushed wood in the for of saw-dust, chips and the like.
Thus, compression-moulding compositions which incorporate a binder in the form of viscous resol phenol-formaldehyde resins or carbamide resins and a filler, viz. crushed wood are known. However, these resins do not ensure a sufficient impregnation of wood. Articles manufactured from such compression-moulding compositions feature a low water-resistance, unsatisfactory appearance and insufficient curing rate. Furthermore, the use of viscous binders adds to production costs of the final articles. Improved impregnation of wood particles is ensured due to a partial replacement of viscous binders with less viscous products of initial condensation of resin-forming monomers (phenolalcohols, urea-formaldehyde condensates).
Increased water-resistance and curing rate of the composition are ensured due to incorporation of fast-curing resins or various additives into the composition. As a curing agent in said compositions use is made of hexamethylenetetramine and as curing accelerators use is made of calcium oxide or magnesium oxide.
A compression-moulding composition based on a water-soluble resol phenol-formaldehyde resin, wood saw-dust, and purpose additive, is also known in the art the ingredients being present in the following amounts; resin 30% by weight, sawdust 67% by weight, oleic acid 0.8% by weight (as a lubricant it is also possible to use stearin, calcium stearate), watersoluble nigrosine 2.2% by weight produced by the known method.
This prior art compression-moulding composition has the following characteristics: water-absorption up to 370 mg, curing rate 1 min/m, hardness 180.0 MPa/m.sup.2. Low water-resistance, absence of gloss, rough surface, non-uniform colour of articles do not make possible to widen the range of application of this composition.
Another disadvantage of the composition resides in high cost of articles produced therefrom due to a low curing rate which results in a longer time of residence of the composition under pressure during processing.
Conventional methods for preparing compression-moulding compositions from phenol-formaldehyde resins and an organic filler comprise intermixing of a binder, a filler and additives, followed by homogenization of the mixture by rolling at a temperature within the range of from 80.degree. to 145.degree. C., cooling and grinding of the material. Total duration of the process is about 2 hours. During hot-rolling of the mixture there occurs, in addition to homogenization, its polycondensation wherefore it becomes difficult to attain a required polymerization degree of the mixture.
This disadvantage may be overcome by separation of said stages, i.e. by performing the stage of homogenization at a temperature of at most 90.degree. C., followed by a heat-treatment of the mixture to a required degree of polymerization.
These prior art methods have disadvantages residing in their multi-stage character (mixing of the components, rolling, cooling, crushing of the rolled material, fine grinding, screening), high rate of power- and labour-consumption, unsatisfactory sanitary and hygienic conditions of labour, especially at the stage of rolling.
Apart from the methods, wherein a filler is combined with a high-viscous binder, known in the art are methods for preparing compression-moulding compositions, wherein the filler is treated with an aqueous solution of phenol, formaldehyde, and an acid at a temperature of up to 300.degree. C. under a pressure of from 20 to 200 kg/cm.sup.2 or by treating a filler with phenol and an aldehyde in the presence of ethylene- and diethyleneglycol employed as a plastifying agent. The process is carried out in an extruder at a temperature within the range of from 130.degree. to 250.degree. C.
Disadvantages of the above-described prior art methods reside in the necessity of performing the process at high temperatures and pressures thus complicating the process technology due to retaining labour-consuming stages necessitating crushing and grinding of the material.
A method known in the art for preparing compression-moulding compositions based on a water-soluble resol phenol-formaldehyde resin, a mixture of saw dust and ground wastes of plywood, wherein fine wood particles are preliminary dried, mixed with the resin, curing agent, dye, and a lubricant in a mixer of the "runner" type. Then the resulting wet mixture is dried at a temperature within the range of from 80.degree. to 90.degree. C. and screened.
This prior art method has a disadvantage residing in a multi-stage character of the process and a long duration thereof (more than 4 hours). Furthermore, the use of rather large wood particles does not ensure high characteristics of a compression-moulding composition.